1. Field of the Invention
The present invention relates to a monomer having a diol structure, a polymer thereof, and a negative photoresist composition and a pattern forming method using the same. More particularly, this invention relates to a monomer suitable as a material for a resist which is used in photolithography in fabrication of various kinds of semiconductor devices using deep ultraviolet rays of 220 nm or shorter, particularly, light from an ArF excimer laser, as exposure light, a polymer thereof, and a negative photoresist composition and a pattern forming method using the same.
2. Description of the Related Art
Higher density and higher integration have been pursued for various kinds of typical semiconductor devices which require micro processing of a half micron order. This leads to an ever growing demand for the photolithography technology to form micro circuit patterns.
One way of achieving micro patterns is to shorten the wavelength of exposure light which is used in forming a pattern using a photoresist. Therefore, a consideration has been given to the use of an KrF excimer laser having a shorter wavelength (=248 nm) in place of i rays (wavelength=365 nm) which have been used so far as the exposure light source in the mass production of DRAMs of 256 Mbits (processing size of 0.25 micrometers or smaller). Further, the fabrication of DRAMs having an integration scale of 1 Gbits (processing size of 0.18 micrometers or smaller) or greater which demands a finer processing technique needs a light source of a shorter length. For this purpose, the use of photolithography using an ArF excimer laser (193 nm) is under consideration.
Photolithography using an excimer laser requires an improvement on the cost performance of the laser because the service life of the gas that is the raw material to cause laser oscillation is short and the laser device itself is expensive. In view of the above, there are increasing demands for higher sensitivity as well as for higher resolution which matches the microminiaturized processing size.
A chemical sensitization resist which uses a photoacid generator as a photosensitive agent is well known as a highly-sensitive photoresist. The feature of the chemical sensitization resist lies in that a protonic acid produced from the photoacid generator containing in the resist by irradiation of light causes an acid catalytic reaction with the base resin or the like of the resist in a heat treatment after exposure. This chemical sensitization resist has achieved an extremely high sensitivity as compared with the conventional resist that has a photoreaction efficiency (reaction per photon) of less than 1. A typical example of the chemical sensitization resist is a positive resist described in Examined Japanese Patent Application KOKOKU Publication No. H2-27660, which consists of a combination of triphenylsulfonium hexafluoroarsenate and poly(p-tert-butoxycarboxy-xcex1-methylstyrene). As a negative photoresist, a resist consisting of a combination of polyvinylphenol and a melamine derivative disclosed in Proceeding of SPIE, vol. 1086, pp. 34-45 (1989) by L. E. Bogan et al.
Resins having a benzene ring, such as novolak and polyvinylphenol have been used for resists for the g rays, i rays and KrF excimer laser. But, resins having a benzene ring show a significant high light absorption with respect to light having a wavelength of 220 nm or shorter, such as the light from the ArF excimer laser. If those resists are used in photolithography using the ArF excimer laser, most of the exposure light is absorbed by the surface of the thin film so that the exposure light does not reach the substrate. This disables the formation of a fine resist pattern. The resins that have been used for resists for g rays, i rays and the KrF excimer laser cannot be adapted for use in photolithography using light with a short wavelength of 220 nm or shorter. With regard to the resists for the g rays, i rays and KrF excimer laser, however, the dry etching resistance that is essential for resists for manufacturing semiconductors are actually acquired from the benzene ring in each resin so that it is not good enough to simply use the benzene ring. That is, resists for exposure with the ArF excimer laser should have an etching resistance without containing a benzene ring and be transparent with respect to the wavelength of 220 nm or shorter.
Active studies are being made on positive resists which have a high transparency to the ArF excimer laser light (193 nm) and have a dry etching resistance. Resins having an alicyclic hydrocarbon group are used as the base resin in those resists. Typical examples include a copolymer having a unit of adamantylmethacrylate (Journal of Photopolymer Science and Technology, vol. 5 (no. 3), pp. 439-446 (1992) by Takechi et al.), a copolymer consisting of adamantylmethacrylate and oxocyclomethacrylate (Journal of Photopolymer Science and Technology, vol. 7, p. 31 (1994) by Takahashi et al.), a copolymer having a unit of isobornylmethacrylate (Journal of Photopolymer Science and Technology, vol. 8 (no. 4), pp. 623-636 (1995) and vol. 9 (no. 3), pp. 465-474 (1996) by R. D. Allen et al.), and a copolymer having a unit of carboxylated tricyclodecyl-methylmethacrylate (Proceeding of SPIE, vol. 2724, pp. 377-385 (1996).
On the other hand, very few studies have been made on negative photoresists for exposure by the ArF excimer laser. The only negative photoresist that provides a high resolution is the negative photoresist consisting of a combination of poly(carboxytetracyclododecylacrylate-hydroxytricyclodecylacrylate), tetrakis(methoxymethyl)glycoluril and triphenylsulfoniumtriflate proposed by the present inventors. Proceeding of SPIE, vol. 3333, pp. 417-424 (1998) describes this resist having a resolution of 0.18 micrometers.
In resins which use alicyclic (meth)acrylate having a hydroxyl group like hydroxytricyclodecylacrylate, the hydroxyl group has both an effect of improving the adhesion to the substrate and a reactivity to a crosslinking agent like tetrakis(methoxymethyl)glycoluril. Therefore, alicyclic (meth)acrylate having a hydroxyl group can be used as the base resin for a negative photoresist without copolymerization of norbornene and maleic anhydride disclosed in Unexamined Japanese Patent Application KOKAI Publication No. H110-10739.
Hydroxytricyclo[5.2.1.02,6]decyl(meth)acrylate disclosed in Japanese Patent No. 2776273, and tricyclodecandimethanolmonoacrylate disclosed in Unexamined Japanese Patent Application KOKAI Publication No. H10-307400 are known as alicyclic (meth)acrylate having a hydroxyl group.
If such negative photoresists have an insufficient crosslinking density, they are apt to be swelled in a developer, resulting in pattern deformation, or stripping of the obtained patterns is likely to occur due to insufficient adhesion.
Accordingly, it is an object of the present invention to provide a negative photoresist which is suitable for use in photolithography using light of 220 nm or shorter like the light from the ArF excimer laser as exposure light, avoids pattern deformation originated from swelling and has a high adhesion strength to the substrate (a micro pattern is hard to be stripped from the substrate) in addition to a dry etching resistance and high resolution.
To achieve the above object, the present inventors have made extensive studies and found that the object could be achieved by a monomer having a diol structure, a polymer thereof, a negative photoresist composition and a pattern forming method using the same.
A monomer according to the first aspect of this invention has a diol structure represented by a following formula (1): 
where R1 is a hydrogen atom or methyl group, R2 is a C2-C6 alkylene group and X is an alicyclic alkyl group having a diol structure.
X in the formula (1) is an alicyclic alkyl group having a diol structure represented by a following formula (2), (3), (4) or (5): 
where n is an integer from 0 to 3, R3 and R4 are a hydrogen atom or methyl group, 
where R5 is a hydrogen atom or methyl group, 
where R8 is a hydrogen atom or methyl group, or 
where R7 is a hydrogen atom or methyl group.
A polymer according to the second aspect of this invention has a diol structure having a repeating unit represented by a following formula (6): 
where R1 is a hydrogen atom or methyl group, R2 is a C2-C6 alkylene group and X is an alicyclic alkyl group having a diol structure.
X in the formula (6) is an alicyclic alkyl group having a diol structure represented by a following formula (7), (8), (9) or (10): 
where n is an integer from 0 to 3, R3 and R4 are a hydrogen atom or methyl group, 
where R5 is a hydrogen atom or methyl group, 
where R6 is a hydrogen atom or methyl group, or 
where R7 is a hydrogen atom or methyl group.
It is desirable that the polymer has a diol structure represented by a following formula (11) and a weight average molecular weight of 1,000 to 50,000: 
where R1 and R2 are the same as those in the formula (6), X is the same as that in any of the formulae (6) to (10), R8 is a hydrogen atom or methyl group, is a C7-C18 crosslinked cyclic hydrocarbon group having a carboxyl group, R10 is a hydrogen atom or methyl group, R11 is a C7-C13 hydrocarbon group having a hydroxyl group, R12 is a hydrogen atom or methyl group, R13 is a hydrogen atom or a C1-C12 hydrocarbon group having a hydroxyl group, and w, x, y and z are arbitrary values satisfying w+x+y+z=1, 0 less than wxe2x89xa61,0xe2x89xa6x less than 1, 0xe2x89xa6y less than 1 and 0xe2x89xa6z less than 1.
A negative photoresist composition according to the third aspect of this invention contains a polymer having a diol structure according to the second aspect of this invention, a crosslinking agent comprised of a compound containing a functional group represented by a following formula (12) and a photoacid generator which generates acid by exposure: 
where R14 is a hydrogen atom, a C1-C6 alkyl group or a C3-C6 oxoalkyl group.
The negative photoresist composition may further contain a polyhydric alcohol compound.
The crosslinking agent may be comprised of at least one of compounds represented by following formulae (13) to (17): 
where R14 is the same as that in the formula (12), a1 is 1 or 2, a2 is 1 or 2, b1 is 0 or 1, b2 is 0 or 1, a1+b1=2 and a2+b2=2, 
where R14 is the same as that in the formula (12), R15 is a hydrogen atom, a hydroxyl group, a C1-C6 alkoxy group or a C3-C6 oxoalkyloxy group, 
where R14 is the same as that in the formula (12), R16 is a hydrogen atom, a hydroxyl group, a C1-C6 alkoxy group or a C3-C6 oxoalkyloxy group, and R17 is an oxygen atom, a sulfur atom, a C1-C3 alkylene group or a hydroxymethylene group, 
where R14 is the same as that in the formula (12), R18 is a hydrogen atom or a methyl group, and 
The photoacid generator may be comprised of at least one of a sulfonium salt compound represented by a following formula (18), an iodonium salt compound represented by a following formula (20), an imide compound represented by a following formula (21) and a diazo compound represented by a following formula (22): 
where R19, R20 and R21 are independently an alkyl-substituted, halogen-substituted or unsubstituted aromatic group, an alicyclic alkyl group, a crosslinked cyclic hydrocarbon group, a 2-oxoalicyclic alkyl group or an alkyl group, Yxe2x88x92 is BF4xe2x88x92, AsF6xe2x88x92, SbF6 or an ion represented by a following formula (19), 
where Z is CnF2n+1 (n is an integer from 1 to 6), an alkyl group or an alkyl-substituted, halogen-substituted or unsubstituted aromatic group, 
where R22 and R23 are independently an alkyl-substituted, halogen-substituted or unsubstituted aromatic group, an alicyclic alkyl group, a crosslinked cyclic hydrocarbon group, a 2-oxoalicyclic alkyl group or an alkyl group, and Yxe2x88x92 is the same as that in the formula (18), 
where R24 is a halogen-substituted or unsubstituted alkylene group, an alkyl-substituted, halogen-substituted or unsubstituted dihydric aromatic group, and R25 is a halogen-substituted or unsubstituted alkyl group, an alkyl group or a halogen-substituted or unsubstituted aromatic group, and 
where R26 and R27 are independently a halogen-substituted or unsubstituted alkyl group, a halogen-substituted or unsubstituted aromatic group or an alicyclic hydrocarbon group.
It is desirable that the negative photoresist composition contains 50 to 98 parts by weight of the polymer, 1 to 50 parts by weight of the crosslinking agent and 0.2 to 15 parts by weight of the photoacid generator.
A resist pattern forming method according to the fourth aspect of this invention comprises a step of applying a negative photoresist composition as recited in claim 7 onto a substrate to be processed; a step of exposing the negative photoresist composition with light having a wavelength of 180 to 220 nm; a baking step; and a developing step.
ArF excimer laser light may be used as the light having a wavelength of 180 to 220 nm.